1

PAPER IV - MEDICINAL CHEMISTRY

UNIT - 1 DRUG DESIGN

1.0 Introduction

1.0.1 Objectives

1.1 Development of new drugs

1.2 Procedures followed in drug design.

1.3 Concept of lead compound and lead modification.

1.4 Concept of prodrugs and soft drugs

1.5 Structure

1.5.1 Activity relationship (SAR), factors affecting bioactivity resonance, inductive

effect.

1.5.2 Isosterism, bio-isosterism, spacial considerations

1.5.3 Theories of drug activity

1.5.4 Occupancy theory, rate theory, induced fit theory.

1.5.5 Quantitative structure activity relationship & History and development of QSAR.

1.6 Concepts of drug receptors

1.6.1 Elementary treatment of drug receptor interactions.

1.7 Physicochemical parameters : Lipophilicity

1.7.1 Partition coefficient

1.7.2 Electronic ionization constants

1.7.3 Steric, Shelton and surface activity parameters & redox potentials

1.7.4 Free-Wilson analysis

1.7.5 Hansch analysis

1.7.6 Relationship between Free-Wilson and Hansch analysis

1.7.7 LD-50, ED-50 (Mathematical deviations of equations excluded)

1.7.8 Let us sum up

1.7.9 Check your progress, the key

1.8 References

1.0 Introduction :

The drug term is derived from drogue-a dry herb, a french word. Drug is present in

medicine i.e. used to prevent and cure of different diseases by treatment. According to WHO

(1966), "Drug is any substance or product i.e. used or intended to be used to explore

physiological systems for the benefit of the recipient.

Essential drugs satisfy the priority of healthcare needs of the public, are intended to be

available of functioning Health systems at all times in excess amounts. Essential drugs is

changing priorities of public health, epidemiological conditions and also the availability of better

drugs, formulations and progress in pharmacology. Adequate data on its safety and efficacy

should be determined by clinical studies. Quality of drug, bioavailability and stability on storage,

safety and price can be assured.

Brand names of drugs are different in different countries. Local action of drug is shown

by tropical route (skin and mucous membranes, deep tissue route and by arterial router). There

are some categories of systemic action, the drug is administered by systemic routes and absorbed

by blood and distributed all over the desired site of areas.

Oral route of drug is safer, convenient, cheaper, painless. In sublingual route drug is kept

under tongue in mouth and spread by buccal mucosa. Rectal route is used when the patient has

vomitting, cutaneous route drug is used as an ointment and applied on specific area of skin.

Others ways are inhalation of drug, nasal route and parental route (subcutaneous injection,

intramuscular, intravenous and intradermal injection).

2

Adverse effects of drugs, any unintended consequence of drug is called adverse effect,

include all types of noxious effects as serious or fatal. All drugs are capable to produce adverse

effects Minimization of adverse effects as injection of aminophylline must intravenously given

slowly, rule out possibility of drug interaction when more than one drug is prescribed, elicit

history of drug diseases and exercise caution, considering previous history of drug reactions, use

appropriate dose, frequency and route of drug administration according to patient's clinical

conditions.

Drug design for our purpose is constituted as the process of envisioning and preparing

specific new molecules that can lead more efficiently to useful drug discovery. Not only did the

rapid development of organic & medicinal chemistry make it possible to determine the structure

of natural drugs.

New drug discovery may be considered broadly in terms of two kinds of investigational

activities "exploration and exploitations" of leads. The former involves the search for a new lead :

the latter the assessment, improvement and extension of the lead & rational approaches to drug

design have been productive. The smaller the expenditure of human and material resources

required to generate a new drug of a particular value, the more efficient the design of the

program. Contributing factors include not only the compounds it was necessary to prepare before

the most satisfactory was found, the medicinal chemists can contribute to the efficiency of

developing or exploiting a lead by making fewest unsatisfactory compounds. The discovery of

useful new drug or lead involves an element of luck. However, the probability of a favorably

manifestation of luck can be improved by taking into consideration the potential for interaction

with living systems through involvement of steric and electronic features of the designed

molecules and of possible biochemical target moieties.

1.0.1 Objectives :

The main aim of this unit is to study the development and procedure of new drugs with lead compound and modification

We also get knowledge about prodrugs & soft drugs.

The knowledge of structure activity relationship (SAR) and factors is important.

Stereochemistry of drugs i.e. isosterism, bio-isosterism with spacial considerations.

The theory of drug activities including occupancy, rate theory & induced fit theory, Drug receptors, physicochemical parameters. Free Wilson and Hansch

analysis & its relationship

We also shall know about doses of drugs LD-50 ED-50 i.e. Lithal and essential doses.

1.1 Development of New Drugs

Hippocrates postulated that, "A disease is pathological process, containing a cause and

nature, and its treatment with medicine is not a magic", based on science. observations, analysis

and deductions. Before this study, it was believed that the cure of disease was mainly based on

the combination of guess work and experience.

In eleventh century Persian scientists Rhazes and Avrienna introduced for cold and cough

and extract of wild autumn crocus (Colchicum) seed for the treatment of gout pain. Both of these

medicines are still used in modern medicine.

The era of synthetic drugs had to wait till the process and techniques of synthetis organic

chemistry became quite advanced and physiology of human body systems was well studied.

During the first half of nineteenth century, ether and chloroform were introduced as

anaesthesia, for the first time as synthetic or organic chemicals.

3

In 1899, aspirin was prepared, it was a result from an· experiment to reduce the nausea

which was caused by the salicylates.

Phenacetin was discovered durin this time, its preparation was resulted from observations

of the hydrolxylation and conjugation of aniline.

Antipyrine was introduced from investiations of the chemistry of quinine.

Poul Ehrlich (1854-1915), who was called Father of Chemotherapy He gave original

ideas about the models of action of drugs. At the age of 45, he was appointed director of the

Institute for experimental therapy in Frankfurt (Germany) in 1899.

In 1891, Ehrlich discovered the antimalarial activity of methylene blue. He further

developed an antibacterial compound acriflavine. Ehrlich also proposed that union between the

alkaloid and the chemoreceptor is labile and reversible and not firmly bound. His further work

with dyes resulted in the discovery of trypanocidal action of trypan red and trypaflavin.

In 1891, the antimalarial activity of Plasmoquine (1926) and Atabrine (932) were

discovered by Ehrlich.

Theory of drug action and the discovery of the sulfonamides and antibiotics were

characterized by increasing knowledge of the chemistry of natural substances specifically

enzymes.

The science of enzymology developed rapidly during post-Ehrlich period. In 1897,

Buchner firstly observed fermentative capacity of cell-free-yeast broth. In 1926, Summer had

crystallized unrease enzyme.

Factors affecting development of new drugs :

(a) Ability of the Chemist : Knowledge about biology of the diseased state of which therapies are being considered. Ability of organize and plan the research project

to get maximum success.

(b) Screening Facility Drugs : Capacity of the screening to evaluate a large number of compounds. The test system which is able to detect potentially and clinically

useful drugs.

(c) Development facility : To develop a new drug, there should be a healthy environment with all physico-chemical facilities, including electron microscope

etc. To investigate the modes of action of bioactive compounds.

(d) Cost of drug development : If the compound is prepared by an expensive process then the cost of manufacturing may also increase dramatically, hence the cost of

drug into market may rise. The number of compounds synthesized in 1958 were

14600, out of them 94 compounds found their way into the market (I in 332

compounds). Similarly in 1964 1,50,000 compounds were synthesized as new

drugs, but only 17 could be marketed.

1.2 Procedure followed in Drug Design :

The development and search of new, safe and effective drugs has become expensive and

costly. A drug does not have and leave any adverse effect on the health as well as on genetic

material affecting the offspring. The drug scientist must create a degree of efficiency in the

synthesis, testing and clinical trial, which will improve the chances for finding new drugs and

also will preserve the resource at their disposal. The information found during this process

describes a structure activity relationship (SAR). For a particular study, there are two steps to a

search for an SAR :

A relationship can be obtained between a systemic structural change in a series of molecules and the observed changes in the biologic activity through the series.

The development of a useful SAR from chemical and biological work is an intellectual exercise. The search for an SAR is a non-experimental part of drug

design and study. This is a theoretical aspect of the drug design process.

4

1.3 Concept of Lead compound :

The drugs used in medicine are developed from so-called lead compounds are also called

Tailor made compounds. "A lead compound is the starting point from which a clinically useful

compound can be developed" or it is starting point when designing a new drug. Lead compounds

are often unsuitable for clinical use because they may be either toxic or have some side effects.

Sources of lead compound is flora (plant) and fauna (animal) e.g. Artemisinin, Venom and toxins.

Micro-organism eg. antibiotics asperlicin, Lovastantin, marine chemistry eg. Curacin-A &

Biochemistry-Epinephrine and histamine.

The search of Lead compound : Suitable tests are required to search for lead compound

i.e. physiological and cellular effect, or the binding of compound & pharmacologically active.

1.4 Concepts of Pro drugs and Soft drugs :

These drugs require a conversion in the body to one or more active metabolites. Such a

drug is called a prodrug.

The prodrugs are more stable, having better bioavailability, less side effects and toxicity,

as well as better other desirable pharmacokinetic properties.

Phases in Prodrug Action

(1) Pharmaceutical (2) Pharmacokinetics (3) Pharmacodynamics

Pharmaceutical Phase

(a) Esthetic Problems : This is a problem of foul odour, appears due to chemical composition of drug. Ethylmercaptan is a useful medicine as antitubercular and

antilepral drug.

(b) Physicochemical Problems : Few drugs some physicochemical problems, e.g. sodium salt of ampicillin in concentrated solution forms polymer of degraded

ampicillin.

Pharmacokinetics Phase :

In this phase the formation of a prodrug undergoes following problems :

Absorption : The drug is poorly absorbed by gastrointestinal tract and other membranes.

Elimination : A drug is eliminated from human body at rapid rate that it can not have the

physiological action in the body over a longer period of time. Metabolism : The drug is

metabolised fastly and converted into inactive metabolites. Toxicity may effect at the site

of administration of the drug.

Pharmacodynamics Phase :

Formation of Prodrugs containing various chemical groups :

(a) Mercaptans : Similar to the alcohols, mercaptans may be formulated as the ester

prodrugs. Mercaptans are more reactive than alcohols, because sulphur atom of

mercaptan is more chemically active than oxygen of alcohols.

(b) The following two types of compounds were prepared as insect repellants, in which the

idea was to liberate the insect propellant known as undecylenic acid CH2 = CH–(CH2)8 –

COOH

(a) The quaternary ammonium group was introduced to act as an "anchoring group". Anchoring group assists to fixing of the molecule onto dermal tissue.

(b) The alkyl group, present in the above compounds would help in binding to dermal tissues and also would assist in the hydrolysis of esters.

5

Configurational Properties :

In an acids reacts with a racemic alcohol, forms an ester, which would be a mixture of

50% of the (+) isomer and 50% of the (-) isomer. When this Easter is hydrolysed by enzyme

esterases gives :

(a) Fast release of the acid, if ester is formed with the (+) alcohol. (b) Slow release of the acid, if ester is formed with the (-) alcohol.

Formation of Amines from Amino Acids :

Levodopa is an amino acid, which is converted into dopamine in the brain by

decarboxylase enzyme. L-dopa is a prodrug while dopamine is its active form.

Double Prodrug :

In the prodrug, there are two potential disadvantages :

(a) The bond between the drug and the carrier portion of the prodrug may be too unstable under storage conditions and also in vivo.

(b) The carrier portion of the prodrug may be inadequate to release the prodrug at the site of action.

Sitespecific delivery of a double prodrug :

movement to site specific

Double prodrug –––––––––––– Prodrug ––––––––– drug

the site of action cleavage

Triple Prodrugs :

In the formation of a triple prodrug, the length of action is increased, since there is a

release of the drug from both the double prodrug and prodrug, cephalosporin was require, and in

particular, the compound should be water soluble.

6

1.4 Soft Drugs

The term 'soft drug' has been derived from the concept of hard and soft acids and bases.

A compound which is easily metabolised, whereas a hard drug means a compound which is hard

to metabolize or is non-metabolisable.

(a) The compound of soft drug is physiologically active. (b) The main aim of preparing soft drugs is not to increase potency, but to increase

the therapeutic indices.

(c) Soft drug avoids the pharmacologically active metabolites. (d) Elimination the drug interactions resulting from metabolite indication of

enzymes, and

TD50

T.I. = –––––

ED 50

where, T.I. = Therapeutic index

TD50 = Median toxic dose

ED50 = Median effective dose

Soft Analogs :

Soft analogs are the bioactive compounds. They are specifically designed that a portion

of the molecule undergoes a checked one stage metabolite process. The metabolism process is to

be encouraged by the hydrolysis process. In the alcohol is released, which decomposed to

formaldehyde and a tertiary amine.

This ester decomposed rapidly. The duration of action was a minute at a concentration of

10-8 M in human plasma. It is absorbed rapidly and then it will be hydrolyzed in vivo. The

structure of this ester resembles to a drug, glycopyrolate.

Activated Soft Compounds :

Activated soft compounds are not traditional analogs of bioactive compounds. Their

design start with a known non-toxic inactive metabolite. An activated group is placed to the non-

toxic inactive molecule, so that in vivo the activated group is released and shows its

pharmacological activity.

Many traditional N-chloramines are very unstable, whereas these compounds liberate Cl+

both inside and outside of the bacterial cell.

1.5 Relationship of Structure and Activity (SAR)

In the nineteenth century, several natural products were isolated and investigated for their

structure and pharmacological activity. It was found that the physiological activity of a compound

is associated with a specific structural group or unit. If such it also contains biological activity.

The part of the compound which is responsible for the actual physiological activity is known as

pharmacophore group.

Factors

1. Effect of Alkyl Groups :

If an alkyl group is introduced in a compound in place of active hydrogen atom,

e.g.,

HCN RCN

ArOH ArOR

RNH2 RNHR

R = alkyl group

(a) Antipyrine is a strong antipyretic, while reduction of a methyl group shows its

inactivity.

7

2. Effect of Hydroxyl Group :

Introduction of hydroxyl group into aliphatic compounds generally decreases their

biological and physiological activity which is almost proportional to the number of the hydroxyl

groups..

(a) Hexanol is more physiologically active then sorbitol.

(b) Butyraldehyde is more active than its -hydroxy derivative.

(c) Propanol is much more active than glycerol.

(d) Hexaldehyde is a toxic compound while its hydroxyl derivative glucose is

physiologically inert.

(e) The physiological action of caffeine is lost in hydroxy-caffeine.

(f) Among the alcohols possessing the same number of carbon atoms, tertiary

alcohol is much more physiologically active than primary alcohol. The order of

their activity is tertiary > secondary > primary.

(g) Salicylic acid consists of antiseptic and antirheumatic properties as compared to

that of inert parent compound, benzoic acid.

(h) Phenol is an antiseptic compound and contains strong toxicity than benzene.

(i) Polyphenols are more toxic in nature than phenol.

3. Effect of Aldehydes and Ketones :

Aldehydes are more reactive than ketones. Their physiological effect is also much more

intense. For example, formaldehyde is an antiseptic compound and it exerts the hardening effect

on tissues. Ketones consist of narcotic action. Their pharmacological properties are similar to that

of secondary alcohols. Aliphatic ketones having alkyl groups, possess the hypnotic activity while

mixed ketones.

4. Effect of Acidic Groups :

Introduction of acidic group in a compound either decreases or totally remove the

biological action of the parent compound. For example :

(a) Nitrobenzene is a poisonous compound whereas its acid derivative nitrobenzoic

acid is harmless.

(b) Phenol is poisonous, but benzene sulphonic acid is harmless.

(c) Morphine consists of strong physiological activity, but morphine sulphuric acid

is completely inactive.

(d) Aniline is toxic while meta-amino benzoic acid has no harms.

(e) Amines are toxic compounds whereas amino acids are used as food-stuffs.

The sequence of various acyl derivatives in the decreasing order of their solubility are as

follows : Lactyl > Acetyl > Benzoyl > Slicyl

Acetyl derivatives are cheap and can be easily hydrolysed, therefore these are generally

more convenient. The benzoyl derivaties are hydrolysed very slowly. The presence of benzoyl

group is of great importance to the physiological activity of ester compounds. The poisoning

effect of tyrosine can be restored by esterification.

The acylation of the compound decreases its basicity. Action of the acylated derivative is

then of immense importance after hydrolysis in the body and thus exerts its physiological action.

8

5. Effects of Halogens :

(a) Positive Halogens : The presence of 'positive halogen' atom in the compound

decreases the toxicity as well as other useful properties.

(b) Negative Halogens : The presence of 'negative halogen' atom generally increases

both the useful and toxic properties. Negative halogen is present at the non-

conjugated position of the compound. It is important to note that increase in

toxicity by the halogenation process is very negligible. It is observed that the

aliphatic fluorocarbons are much less physiologically active than the other

halogens and even less than the corresponding non fluorinated compounds.

6. Effect of Nitro and Nitrite Groups :

(a) Nitro Groups : The introduction of nitrogroup into an aromatic compound makes

it more toxic.

(b) Nitrite Groups : Physiological activity increases.

7. Effect of Amino Group :

Amino group is toxic in nature. Alkylation reduces their toxicity. Acylation also

decreases the physiological action of parent compound. For example, aniline is physiologically

toxic while acylated derivative, acetanilide is an important fabrifuge. However, aromatic amines

and hydrazines are the compounds possessing analgesic and antipyretic properties.

8. Effect of Nitrile Group :

For KCNS (potassium thiocyanate) is a weak poison while Na2Fe(CN)5 - No (sodium

nitro prusside) is strong poison and even causes death.

9. Effect of Unsaturation :

The toxicity of the compound increases with increasing unsaturation. Alkyl alcohol (CH2

= CH – CH2OH) consists of strong poisonous properties whereas saturated compound, propanol

(CH3CH2CH2OH).

10. Effect of Isomerism :

The isomerism also plays an important role in the field of physiological action of drugs.

For example, the ordinary cocaine is a well known anaesthetic compound while its structural

isomer -cocaine does not have this property. Similarly, sulphanilamide is a very active sulpha

drug, whereas its other two isomers are inactive in nature.

(a) The natural l-adrenaline is twelve times active than its dextro isomer. (b) l-nicotine is two times more poisonous than d-nicotine. (c) dl-hyoscyamine (atropine) is more active than l-hyocyamine.

1.5.2 Isosterism

The concept of isosterism is credited to Langmuir in 1919. He stated that the atoms,

groups, radicals and molecules which have similar physicochemical properties and similar

electronic structure, are known as 'Isosters' and this phenomenon is called "isosterism". Such

similarities occurred in atoms which are in the same vertical column of the periodic table, where

the outer shell of the electrons are identical or almost identical.

In a horizontal row of periodic table, recognized for contiguous atoms. Chemical

properties of chlorine and bromine are more similar than those of carbon and chlorine or chlorine

and iodine. Chlorine consists to 35.46 atomic weight and 1.80 A radii, while iodine have 126.91

and 2.15 A.

9

Physical Properties N2O CO2

1. Viscosity (at 20°C) 148 x 10-6

148 x 10-6

2. Density (at 10°C) 0.856 0.858

3. Refractive index of liquid, D line 16° 1.193 1.190

4. Dielectric constant at 0° 1.593 1.582

5. Solubility in alcohol at 15° 3.250 3.130

In 1925, Grimn has given a set of hydride displacement rules. He formulated that the

vertical columns of isosteric groups were formed by displacing one place to the right the elements

of a horizontal row and adding a hydrogen atom i.e., a hydride ion and continuing this process.

The example is illustrated below where each vertical column represented a group of isosteres and

the process is continued with the next horizontal row of elements.

1.5.2 Bioisosterism

Bioisosteres are the isosteric compounds which have the same type of biological activity.

In 1951, Friedman coined the term biosterism and since then the meaning of the term has

gradually broadened.

(a) Classical Bioisosteres

(i) Monovalent atoms and groups

(ii) Divalent atoms and groups

(iii) Trivalent atoms and groups (iv) Tetra substituted atoms (v) Ring equivalents

(b) Non-classical bioisosteres

(i) Exchangeable groups

(ii) Rings versus noncyclic structures

The monovalent bioisoteres consist of the halogens and the group-XHn, where n=C, N, O

and S.

The divalent atoms and groups included the R-O-R', R-NH-R, R-CH2-R', and R-Si-R'.

The trivalent bioisoteres comprise C and N in the formation of trivalent groups, e.g., R-

N=R' and R-CH=R'.

The tetrasubstituted atoms are limited to three elements = C = C, = N =, and = P =.

(1) True bio isosteres :

Such types of bio sciences have the same type of bioactivity as the analog and in addition

have the same magnitude of the biological response.

(2) Partial bio isotere :

The partial bio isoteres have the same type of bioactivity but has different magnitude of

response. The partial bio isostere are useful if desired bioactivity is retained and reducing the

magnitude of undesired side effects, the modification of some ethylenediamines.

Application of Recent Bioisosterism :

Sulfonamido isosteres of the catecholamines is an excellent example of exchangeable

groups. An alkyl sulfonamido group may be substituted for the phenolic hydroxy group of some

catecholamines. Few of the resulting compounds possess agonist activity, while others are

antagorists. Between catecholamine and alkyl sulfonamidophen ethanol amines, there may be

some analogies.

10

Alkyl sulfonamidophene ethanotamine & phonylephrine. Above compounds consist of

the same bioactivity when administered intravenously, 0.004 mg/kg of the alkyl sulfonamido

compound and 0.002 mg/kg of phenylephrine cause a 20% increase in blood pressure.

Eletro negativity, polarizability, vander wall's radii, bond angles, charge, number of

substituents, acidity and basicity of the atom can highly influence the physiochemical

characteristics of the molecule. The drug molecules exert their effect by affecting receptor sites in

living system, via their physicochemical properties. It is also notable that change the physico

chemical properties of the molecule, and thus causes the biologic response to it.

1.5.3 Theories of Drug Activity

The drug action is either due to their physiochemical properties, as in the case of

structurally non specific drugs or from their chemical structure, as in the structurally specific

drugs. The structurally specific drugs act in very small doses, and their activity arises from

complexation with specific receptors localized in molecules of the organism. The structural non

specific drugs aliphatic alcohols, act in large doses by forming a monomolecular layer over the

whole area of certain cells of the organism.

1.5.4 Occupancy Theory :

This theory was developed by Clark and Gaddum, and also called template theory which

states that the intensity of certain pharmacological effect has been directly proportional to the

number of receptors occupied by the drug. According to occupancy theory, drug receptor

interactions, comply with the law of mass action, may be shown by the following equation.

where R = receptor and D = a molecule of the drug

RD = drug-receptor complex

E = pharmacological effect and k1, k2 are rate constants of absorption and desorption

respectively.

The number of occupied receptors depend on the concentration of the drug in the

compartment of the receptors and also on the total number of receptors in the unit are or volume.

Affinity and Intrinsic activity :

In opposite to the occupancy theory not all agonist of a given class of drugs elicit the

same maximal response an example is the alkyltrimethyl ammonium series of acetylcholine

congeners. This theory fails to explain some drugs act. They stated that drug-receptor interaction

involves two stages :

(a) Complexation of the drug with its receptors. (b) Production of effect. Localization of some drug-receptors or acceptors has been determined. Most of them is

either active sites or allosteric sites of enzymes or parts of DNA or RNA. Certain drugs act either

by intercalation between DNA base pairs, as in the case of chloroquine or by alkylating or cross-

linking of DNA strands as in the example of mitomycin.

1.5.4 Rate Theory :

The rate theory depends on the basis that a drug is efficient only at the moment of

encounter with its receptor.

11

Paton stated that activation of receptors is proportional not to the number of occupied

receptors but to the total number of encounters of the drug with its receptor per unit time. The rate

theory does not need formation of a stable Michaelis-Menten complex for activation of the

receptor by a drug. According to this theory, pharmacological activity has been a function only of

the rate of association and dissociation between molecules of drug and receptor and not of

formation of a stable drug-receptor complex. Each association forms a quantum of stimulus for

the biological interaction.

In the example of agonists, the rate of association and dissociation is faster and produce

many impulses in unit of time.

The rate theory is not able to explain various experimentally given facts. For example, the

agonist has characteristics that favour the formation of a complex which does not quickly

dissociate.

The rate theory as well as the occupancy theory have been widely criticized because these

fail to explain the interpretation of phenomena which found at the molecular level, why a drug

acts as an agonist or antagonist.

Paton and Rang gave an alternative to dissociation theory. In their efforts, the

dissociation rate constant has been a function not of the intensity of the binding forces but of the

extent to which the drug molecule disturbs the secondary protein structure. The dissociation

theory has not been formally different from the occupancy theory in relating stimulus to rate of

dissociation, and this rate being proportional to the occupation of receptors.

1.5.4 Induced Fit Theory :

The induced-fit theory is based on hypothesis, for which recent evidence is being

accumulated of induced conformational changes in enzymes. Koshland postulated that the active

site of an isolated crystalline enzyme does not have a morphology which is complementary to that

of the substrate as a kind negative to it. It acquires such morphology only after interacting with

the substrate, which induces a conformational change. The active sites of the enzyme is flexible,

and not rigid, i.e. not only it can be deformed or change but it also consists of the ability to return

to the original form. The induced-fit theory states that the biological effect produced by drugs

arises due to activation or deactivation of enzymes, or of non catalytic proteins, through a

reversible perturbation or alter in tertiary structure of enzymes or proteins. Conformational

change does not get restricted to proteins. Drugs having flexible structure can also undergo

conformational change as they approach the site of action or the receptor site. Drug receptors

interaction can be seen as a dynamic, and reversible, which gives rise to the biological effect.

Koshland et al. have modified the induced fit theory to explain co-operative effects.

Binds one ligand molecule accelerates binding of subsequent ones.

1.5.5 Quantum Mechanical Approaches : History & Development of QSAR

Quantum mechanics or wave mechanics explains a description of matter which is based

on fundamental assumptions of natural phenomena.

Matter is composed of atoms, molecules, protons, neutrons and electrons. Quantum

mechanics must successfully describe properties of these fundamental particles. Electrons of

molecules are involved in the chemical changes, therefore, these are most important particles to a

drug scientist. This theory is a natural extension of the notion of atomic orbitals.

Chemical reactions are governed by the probabilities of finding an electron in a particular

place in the molecule and by the energy of that electron. Molecular orbital theory provides the

calculations on drug molecules, electron location probabilities and energies.

The introduction of the Hansch Model in 1964 was a starting point of QSAR.

(1) In 1968 Crum-Brown and Fraster examined the neuromuscular blocking effects of a

variety of simple quaternary ammonium salts and quarternized alkaloids in animals.

12

From these studies they concluded that the physiological action of a molecular

was a function of its chemical constitution.

(2) Richardson noted that the hypnotic activity of aliphatic alcohol was a function of their

molecular weight.

These observation were the basic of QSAR.

Quantitative Structure Activity (QSAR) represents an attempt to correlate structural or

property descriptors of compound with activities. These physico-chemical descriptors, which

include parameters to account for hydrophobicity topology, electronic parameters and steric

effects are determined empirically or more recently by computational methods.

QSAR is defined by physical properties Intrinsic properties and Biological properties.

The introduction of Hansch method in 1964 enabled chemists to describe SAR studies in

quantitative terms. Methods used in QSAR analysis can be summarised follows :-

(a) Hansch Method : linear free energy relationship

(b) Free Wilson model

1.6 Drug Receptors

Nature of Drug Receptors :

Consist of biological activity even in much less concentrations. These drugs are termed as

structurally specific drugs. A chemical must first with semi-rigid macromolecule, which performs

a biological function. This macromolecule may be an enzyme or may contain a 'receptor'.

(a) A receptor is a marcomolecule bearing sites. It possesses chemorecognitive properties for a specific natural endogenous molecule or for specific drugs.

(b) The specificity of the sites on the receptor macromolecule and the function for a particular endogenous molecule is genetically determined.

(c) Binding of agonists, either the endogenous molecule or a drug, causes a specific perturbation or change in state of the receptor macromolecule.

(d) The initiation of a response by binding at a receptor sites does not depend on the making or breaking of covalent bonds in the agonist.

(e) The destructive toxins develop their injurious action on the cell by the fact that they are absorbed by certain specific component parts of the cell side chains,

which I have characterized as 'receptors'.

a. High Potency : Many drugs act at very low concentration, i.e. 10-9 M and 10

-11 M.

b. Chemical Specificity : The differences in effects produced by optical isomers. Only one of the four isomers of chloramphenicol has been

active.

c. Biological specificity : Epinephrine which has marked effect on heart muscle but very weak action on striated muscle.

That receptors are localized in macromolecules most of which

have protein like properties and have the specific ability to interact with

natural substrates at their active sites. Nature has been probably similar

to the active site of enzymes, and they approximate equal in size the drug

molecule which is able to form a complex with them. A drug converted

to a receptor depend on the structural, configurational and

conformational characteristics of both drug and receptor.

1.6.1 Receptor Interaction :

One of the most useful bits of information that can be obtained from SAR studies is the

types of atoms and functional groups that are important in binding a drug to its target binding site.

There are various forms of bonding that can take place. Usually these are intermolecular bonding

13

interactions such as ionic bonds, hydrogen bonds, van der waals interactions and dipole-dipole

interactions. However, some drugs may form covalent bonds to their targets.

Direct Method :

In the direct method, the functional groups of a receptor with substance able to bind

irreversibly, i.e. by covalent bonding, and then to isolate the resultant drug-receptor complex.

Among the chemical reagents used, can react with the serine hydroxyl group, phosphorylating

agents, sulfonyl flourides, carbamylating agents, alkylating agent, and N-alkyl maleimides as

given below :

Indirect Method :

In the indirect method the macromolecule has the receptor through the use of substances

able to complex with it reversibly.

1.7 Physico-chemical parameters :

It seemed that the compound was effective in man only when potentiated with anesthetics

& the trend to do without deep anesthesia appeared to make the compound impractical as a drug.

As an interesting sidelight, it was conjectured that the presence of two lipophilic groups in the

molecule might result in bonding of the agent to lipophilic sites of loss in the body, and that prior

administration of lipophilic aesthetics might potentiate the blocker by masking these sites of loss.

In a drug series with an ionizable functional group there is some relationship between

biological activity and ionization. Optimum range in pka values for eliciting the biological

activity. Quaternous ammonium salts may be completely ionized, the electron or charge

distribution can vary among different structures and materially alter activity. Phenolic, mercapto

and enolic gps with H-bondig may be more significant.

Change or electron transfer complex formation has attracted attention as a possibly

important biochemical and drug bonding. By Resonance delocalized, IT-electron cloud in

aromatic & hetero aromatic ring systems molecules that are favorable to a receptor. It may be

quite common for drug molecules to induce conformational changes at receptor areas.

1.7.1 Molecular Orbital Indices : Molecular orbital calculations provide useful information

and can give numerical indices which reflect the probable position of an electron and its

energy in a molecular orbital.

Charge : In a molecular orbital, a wave function , is stated to contain a three-

dimensional coordinates of an electron. The wave function of the molecular orbital is assumed to

be a linear combination of values from the contributing atoms in a molecule. The contributions

are designed by coefficient, C. It gives an expression for a molecular orbital of a molecule of n

atoms.

= Ca a+ Cb b+ ...... Cn n

As there are only two electrons in a molecular orbital, there must be many molecular

orbitals in a drug molecule to accommodate the large number of electrons. By summing the

individual 2C2 values of each molecular orbital to obtain electron probability or electron density

in any part of the molecule.

qi = 2C2i

The value of q may deviate due to the nature of atom, i, and a simple count of the number

of electrons contributed by atom i, to the molecular orbital.

Ionization energy E, is related to the wave function , of the molecular orbital.

According to Schrodinger equation :

H = E

where H = Hamiltonian operator

14

A SAR method was developed by Kier and Hall. This method is much less complex than

quantum mechanics, and known as molecular connectivity, which is useful in structural

description.

1.7.2 Electronic Ionization constants :

When ionization of a molecule takes place the hydrophilicity also increases and hence

true partition coefficient of molecule may be determined by assuming that the ion is found

exclusively in the aqueous phase.

P = Coctanol / Cwater (1-)

P = partition coefficient, Coctanol concentration of the compound is 1-obtanol, Cwater =

conc. of compound in water. = degree of ionization of compound.

1.7.3 Steric factors :

Taft steric parameters : Meyer 1895, postulated that the Atomic weight of o-substituent

determined the ease of esterification of o-substituent of aromatic acids, Taft defined numerically

the steric constant Es as an equation :

Es = log (kx/kH) A

k = rate constant for acid hydrolysis of esters of type X-CH2COOR

1.7.3 The redox potential of a system may be calculated from the equation :

0.06 conc. of reductant

E = E' – –––– [ –––––––––––––––– ]

n concn. of oxidant

E = redox potential studied

E' = standard potential at given pH

n = number of electrons transferred

Surface activity : for surface active agents, while they exist as manomers in dilute

solution. They form polymers as the conc. increases and the concentration at which polymers

form is called Critical Micelle Concentration (CMC), since the polymers are micelles. Advantage

of using surface active agents is that the polymers can solubilities of water insoluble compounds

e.g. lipophilic phenols. Thus in evaluating the antimicrobial activity of 4-benzylphenol in the

presence of sodium lauryl sulphate, 3 zones of antibacterials are noted, Antibacterial activity

enhances the antibacterial activity of 4-benzyl-phenol.

1.7.4 Empirical Fragment Evaluation : Free and Wilson Analysis

Free and Wilson developed an approach to structure-activity relationship. In this

approach, changes in biologic activity within a series of related molecules can give numerical

values on a logarithmic scale.

Ban and Fujita studied the action of a series of phenethylamines on the isolated heart.

If A0 = activity of the parent skeleton

and A = activity of any particular molecule in a series,

the model can be expressed as :-

where X = 1 or 0, according to the feature or substituent,

Gi is absent or present. Substituent may be present in ring or chain and compound may be

optically active or racemic.

Log a = Gi Xi + k

Thus for ± Noradrenaline, the equation is in the form :

15

The 'best-fit' value for all G2 and k are solve by computer. The G value represent the

contribution of a particular feature or substituent to activity. Comparison of G values allows one

to discern important features. A more potent molecule may be designed by using substituents

with large positive G values.

1.7.5 Hansch Analysis

This method seeks relationships between biological activity and common physical

properties e.g. degree of ionization, molecular size or lipid solubility. In this method biologic

activity are explained in terms of physical model. Correlations can be made between biologic

activity and a linear combination of indices (parameters).

The activity of a drug molecule can be related to the probability p. A drug going through

the three above steps, a, b, c thus related to the probability of its going through the individual

steps :

p = papbpc

If k = proportionality constant and

c = molar concentration then,

Activity = k . c . p

= k . c . pa pb . pc

log 1/c = log pa + log pb + log pc + k

DOSE, C | k| Response

random walk receptor

log 1/c = k1 log pa + k2 log k + k3

Here, we have expressed.

Intrinsic activity and linear free energy relationship : If a drug is log pa = 0. As a model

the equation is :

log 1/c = k1 log k + k2

Sulfonanilides : Antibacterial Effect : In sulfonamide drugs, the effect of the substituents

R was parameterized by . Between and potency, a good relationship was found.

Expanding above equation to include hydrophobic bonding, a general model equation in

vitro can be written as :

log 1/c = k1 + k2 + k3

1.7.6 Relationship between Free Wilson and Hansch Analysis (Mixed approach) :

Kubinyi has presented the combination of Hansch and Free-Wilson models as mixed

approach. The mixed approach can be written as log 1/c = aij + kjj+k parameters. In this

equation aij is the Free-Wilson part for the substituents and j= and Es contribution of the

present skeleton. The mixed approach was developed to find possible interactions between Free-

16

Wilson parameters and physicochemical properties of the substituents used. Another advantage of

this equation is that the symmetry equations need not to be develop. The reduction of the matrix

is done by setting the increments of the substituents of one chosen reference compound equal to

zero.

The basic assumptions for the use of the Free-Wilson approach are :

(a) The approach can be applied to a congeneric series having a common skeleton.

(b) Various derivatives must have been prepared by using different substituents at the same

distinct positions of the parent skeleton.

(c) When choosing derivatives for the synthesis, care has to be taken that every substituent

appears at least twice at the same position.

(d) It is stated that number of derivaties for the solutions of the regression analysis must be at

least ten, equal to the number of increments. To reduce the number of compounds to be

synthesized. Free and Wilson have proposed a symmetry condition where the sum of

increments in a substitution position was considered equals to zero.

1.7.7 LD50 and ED50 :

The medicinal value of the drugs is generally represented by 'therapeutic index' or 'safety

margin'. Therapeutic index is described as the ratio of the amount necessary to kill the patient

[i.e., median lethal dose (LD50)] to that required for a median effective dose (ED50). In

experimental animals, therapeutic index is calculated as :

Median Lethal dose

Therapeutic Index = –––––––––––––––––––

Median Effective dose

LD50

= –––––

ED50

A therapeutic index often means that ten times a dose used for effective purpose would

kill the patient as well as parasite.

The minimum dose to cause death of 50 percent animals is called LD50. This dose in

general is also known as tolerance or threshold dose.

Similarly, the minimum concentration required to have positive response from 50 percent

of animals is known as ED50, i.e., the effective dose for 50 percent with standard.

LET US SUM UP

After going through this unit, you would have achieved the objectives and learnt about

these stated earlier in the unit. Let us remind/recall what we have discussed so far.

Drug is present in medicine i.e. used to prevent and cure of different diseases by treatment. Essential drugs satisfy the priority of healthcare needs of the public.

Brand names of drugs are differ in different countries.

Oral route of drug is safer, convenient, cheaper and painless than inhalation by nasal route & parental route.

New drug discovery may be considered broadly in term of exploration sand exploitation. The discovery of useful new drug or lead involves an element of

luck. In a drug series with an ionizable functional group there is some

relationship between biological activity sand ionization.

Theory of drug action and the discovery of the sulfonamides and antibiotics were characterized by increasing of chemistry of natural products specially enzymes.

17

The development and search of new, safe and effective drugs has become

expensive and costly.

Structure Activity Relationship (SAR) is information found during the process. Computers are used in drug designing and development.

Prodrugs and derivatives of bioactive molecules which are inactive and converted invivo to the drug. The term soft drugs is taken from the concept of Hard and

Soft acids and bases. Thus a hard drug is hard to metabolise and is non-

metabolisable while soft drug is easily metabolised.

Free and Wilson analysis used for structure activity relationship and in Hansch analysis we came to know relationships between biological activity and common

physical properties i.e. degree of ionization, molecular size or lipid solubility.

Quantitative Structure Activity (QSAR) represents an attempt to correlate

structural or property descriptions of compound with activities.

The concept of receptor is most useful information that is obtained from SAR studies, is the types of atoms and functional groups that bind a drug to its target

binding sites.

Occupation theory : shows that different agonists action on the same receptor system do not give identical maximum effects. This theory lead to the

introduction of the terms like intrinsic activity sand affinity as two different

parameters from drug action unlike that just affinity as proposed in clark's view.

Biological effect = intrinsic activity x drug receptor complex

Induced fit theory postulated that the active site of the enzyme is flexible or better, plastic or elastic and not rigid.

Lipophilicity of a drug can be measured readily by distribution of the compound between an aqueous and non-aqueous, water immissible solvent. The non-

aqueous solvent used is l-octanol.

The Hancsh value indicates the contribution of a substituent to the hydrophobicity and hydrophilicity for different bioactive compounds.

= log (Px / Pu)

Steric factors (Taft steric parameters : Es), Meyer postulated that atomic weight of O-substituent determined the ease of esterification of O-substituted aromatic

acids.

Es = log (Kx / KH) A

Ions are found in Aqueous phase.

When surface active compounds is placed in water, it will align itself so that polar group in water & the non-polar functions is placed vertically above water.

The surface active agents, exists as monomers in dil solution. They form

polymers as the concentration increase & form the polymers at concentration is

the critical Micelle concentration (CMC), since the polymers are micelles.

18

CHECK YOUR PROGRESS : THE KEY

1. (a) Development of new drugs

(b) Drug design

(c) Concept of lead compounds

(d) Activity relationship (SAR)

(e) Isosterism

(f) Spacial considerations

2. (a) Theories of drug activity

(b) Occupancy theory

(c) Rate theory

(d) Induced Fit theory

(e) Free & Wilson analysis

(f) Hansch analysis

(g) Concept of Pro & Soft drugs

3. (a) Applications of bioisosterism

(b) How the drug activity increased

(c) LD50 and ED50 - Lethal dose & effective dose

(d) How the esterification & chlorination affect the activity of drug ?

(e) Lack of literature & knowledge of drugs.

1.8 References :

A. Burger, Medicinal Chemistry, 3rd ed. New York, Willey-interscience.

Alka L. Gupta, Medicinal Chemistry, Pragati edition, Meerut.

E.W. Gill, Progr. Med. Chem. 4, 39 (1965).

G.R. Chatwal, Medicinal Chemistry, Himalaya Publ. House, 2002.

K.D. Tripathi, Essentials of Medicinal pharmacology, 5th ed., Japyee brothers Med. Pub. 2003.

M.L. Gangwal & S. Baghel, Drug design & synthetic drugs, Student publishing house, Old Palasia, Indore.

R.E. Thomas, Cardiac Drugs in Burger's Medicinal Chemistry, 4th ed., New York, John Wiley & Sons, 1981.

W.O. Foye, Principles of Medicinal Chemistry, Varghese Pub. House, 3rd ed., pp. 189-221, 2003.

19

UNIT-2

PHARMACOKINETICS & PHARMACODYNAMICS

2.0 Pharmacokinetics

2.0.1 Objectives

2.1 Introduction to drug absorption,

2.2 Disposition

2.2.1 Elimination

2.2.2 Pharmacokinetics of elimination

2.3 important pharmacokinetic parameters in defining drug disposition and in therapeutics.

2.4 Mention of uses of pharmacokinetics in drug development process.

2.5 Introduction of pharmacodynamics.

2.5.1 Elementary treatment of enzyme stimulation.

2.5.2 Enzyme inhibition.

2.5.3 Sulphonamides.

2.5.4 Membrane active drugs.

2.5.5 Drug metabolism.

2.5.6 Xenobiotics.

2.5.7 Biotransformation

2.5.8 Significance of drug metabolism in medicinal chemistry.

2.5.9 Let us sum up

2.6 Check your progress : the key

2.7 Reference

2.0 Pharmacokinetics

2.0.1 Objectives : The aim of this unit is to study the drug movement inside the outside the body. We also

get knowledge about Pharmacokinetics, the quantitative study of drug movement inside, through

and outside of the body is done. We also studied drug absorption by different ways, Distribution

& disposition of drugs, excretion and elimination of drugs & pharmacokinetics of elimination and

different pharmacokinetics in drugs development process has also studied.

We will get knowledge of Pharmacodynamics that deals enzyme stimulation, enzyme

inhibition, sulphonamides, membrane active drugs, drug metabolism, xenobioties &

significances of drug metabolism in medicinal chemistry .

2.1 Introduction

Pharmacokinetics is the quantitative study of drug movement inside, through and outside

of the body. Therefore, pharmacokinetic considerations determine the routes of drug

administration, dose, time of peak action duration of action and frequency of administration of

drug. drug movement occurs through the membranes. Biological membrane is a bilayer, 100 A in

thickness made up of phospholipid, cholesterol, polar groups such as glyceryl phosphate attached

to ethanol amine/choline or hydroxyl group of cholesterol. Polymeric sugars, amino sugars and

sialic acids are attached on the surface and formed glycoprotein or glycolipids. The protein

molecules are able to freely float through the membrane and some proteins are fixed and present

in the full thickness of the biological membrane, which are also surrounded by fine aqueous

pores. The movement of drugs across the membranes occur by following processes

(A) passive diffusion (B) filtration (C) Specialized transport The drug diffuses across the biological membrane in the direction of its concentration

gradient. The drugs which are soluble in lipids, diffused by dissolving in the lipoidal matrix of the

20

biological membrane. Highly lipid soluble drug attains higher concentration in the membrane and

diffuses quickly.

Generally most of the drugs are weak electrolytes and their rate of ionization depend on pH of

drugs. They in the following manner :

(1) Acidic drugs, e.g., aspirin, whose pka value is 3.5, are pH of gastric juices. These drugs are absorbed by stomach.

(2) Basic drugs, e.g., atropine, whose pka value is 10, are largely ionized. These are absorbed only when they reach in the intestine.

(3) Unionized form of acidic drugs cross the surface membrane of gastric mucosal cell. These drudgs also revert to the ionized form within the cell

(pH 7.0) than slowly pass to the extracellular fluid. This is called ion

trapping of drug.

(4) Basic drugs attain higher concentration intracellularly. (5) Acidic drugs are rapidly ionized in alkaline urine. They do not diffuse

back in the kidney tubules and are excreted quickly.

(6) If urine is acidified, basic drugs are excreted faster.

(B) Filtration : Drugs filtration occur through aqueous pores of the membrane or through

paracellular spaces. It is faster when osmotic pressure gradient is available. Most of the cells,

e.g., RBC, intestinal mucosa etc. consist of very small pore size, i.e., 4 Å, and drugs with

molecular weight more then 100 or 200 are not able to penetrate them. Drugs of larger molecular

wt. e.g. albumim can filter through capillaries depend on rate of blood flow.

(C) Specialized transport : This is of two types :

(a) Carrier transport

(b) Pinacytosis

(a) Carrier transport : In the carrier transport system, a drug combines with a carrier which is present in the

biological membrane and forms a complex.

(i) Active transport : This transport system need energy and occurs against the

concentration gradient. It gets inhibited by metabolic poisons.

DRUG ABSORPTION

Absorption of drug is the movement from its site of administration into the circulation,

When given intravenously, the drug has to cross the biological membrane. The absorption of

drugs is governed by the above described principles. The factors affecting the absorption are as

follows :

(a) Aqueous solubility : If drug is solid, it is necessary to dissolve it in aqueous biophase before absorption. A drug given as watery solution is absorbed faster

than when the same is given in solid form or as oily solution.

(b) Concentration : Drug given as concentrated solution is absorbed faster than from dilute solution.

(c) Surface area : if surface area of drug absorption is larger, it means faster absorption.

(d) Vascularity of the absorbing surface: circulation of blood removes the drug from the site of absorption and maintains concentration gradient across the

biological membrane.

21

Route of Adminsitration : Route of administration affects the absorption of drug.

(a) Oral route : Drug which are taken orally, absorb in the following manner : (i) Nonionized lipid-soluble drugs : are quickly absorbed by stomach and

intestine.

(ii) Acidic drugs : salicylates and barbiturates etc. are acidic drugs which are unionized in gastric juices. These drugs are readily absorbed by stomach.

(iii) Basic drugs : Quinine , morphine etc. are highly ionized drugs. These are absorbed only in duodenum.

(iv) Solid drugs : Drugs which are given in the form of solid dosage are governed by rate of dissolution and rate of abortion.

(v) Absorption in presence of food : Presence of food reduces the absorption of drugs. some drugs form an complex compound with food constituents,

e.g., an antibiotic "tetracycline", form a complex with a calcium which is

present in milk. These most of the drugs absorbed better when taken in

empty stomach condition.

(vi) Ionized drugs : Drugs e.g. neostigmine, streptomycin etc. are highly ionized in nature and are poorly absorbed when given orally.

(vii) Degradation of drugs by gastric juices : Insulin is a drug which is degraded by peptidasa enzyme of gastrointestinal tract, if taken orally.

Therefore, it is administered intramuscularly, Similarly penicillin G is

degraded by acid, and is also ineffective orally.

(viii) Luminal effect of drugs : If two drugs are taken together, they may form an insoluble complex, this known as Luminal effect of drugs. Hence, to

minirmize this effect, two drugs must be taken at 2-3 hour intervals.

Example of such drugs are phenytoin wih sucralfate and tetracycline with

antacid and iron preparations.

(ix) Gut –flora changing drugs : (x) Gut wall effects

(b) Subcutaneous and Intramuscular :

Absorption of drugs from subcutaneous site is slower than from intramuscular site, but

routes are generally faster and consistent than oral absorption. Application of heat and muscular

exercise accelerate drug absorption by increasing blood flow.

By these routes the drug deposited directly in the capillaries. The capillaries are highly porous

and they do not obstruct absorption of large lipid –insoluble molecules or ions. Extremely large

molecules are absorbed through lymphatics.

(c) Topical sites administration :

This type of drug administration occurs though skin, cjornea and mucous membranes and

depends on lipid-solubility of drugs. Only few drugs such as estradiol, hyoscine, clonidine and

nitroglycerine can significantly penetrate intact skin.

Cornea is permeable to lipid –soluble, unionized physostigmine but not to highly jonized

neostigmine.

Abraided surface of skin quickly absorbs drugs. If tannic acid is applied over burnt skin

surface, causes a side effect, 'hepatic necrosis'. Similarly corticosteroids applied over skin can

produce systemic effects and pituitary –adrenal suppression. Organophsophate insecticides

coming in contact of skin can cause systemic toxicity effect.

22

2.3 Distribution And Disposition Of Drugs

After administering the drug in the blood stream, it is ready to distribute to other tissues.

Distribution of drugs depend upon its (a) solubility in lipid; (b) differences in regional blood flow

(c) binding to plasma and tissue proteins (d) ionization at physiological pH.

The distribution of drug continues till an equilibrium occurs between unbound drug .

Apparent volume of distribution (v) : If the body is homogenous single compartment of

volume v, where drug gets immediately and uniformly distributed,

Intravenous dose administered

V = –––––––––––––––––––––––––

Plasma concentration

"V is the volume which would accommodate all the drug in the body if its concentration

is same as in plasma."

Penetration of drug into brain and cerebro spinal fluid : In the brain blood capillaries do

not contain large inercellular pores and have tight junctions. Neural tissues cover the capillaries

of endothelial cells in brain. Thus they form a 'blood-brain barrier'. Choroidal epithelium tissues

also line the capillaries and form a similar 'blood-cerebro spinal fluid barrier'.

Efflux carrier such as P-glycoprotein present in brain capillary endothelial cells, exrtude

several drugs which enter in brain by other processes. Dopamine doses not enter into brain but its

precursor levodopa can panetrate. In the capillary walls or cells lining of brain monoamine

oxidase, cholinesterage and some other enzymes are present, they also form an 'enzymatic blood

brain barrier, this barrier does not permit acetylcholine, catecholamines, 5-hydroxy tryptamine to

enter into brain in active form.

Passage across placenta : Placental membranes are lipoidal and permit free passage only

to lipid-soluble drugs. But when non lipid soluble drugs are taken in high concentration and/or for

long periods by mother, it is gained by foetus. Thus it is an incomplete barrier and almost any

drug administered by the mother can affect the new born.

Plasma protein binding : Acidic drugs bind to plasma albumin and basic drugs bind to

1 acid glycoproteins. Extent of binding dependson the individual compound e.g.,

sulphamethaxine binds 30% sulphadiazine, 50% sulphamethoxazole 60% and sulfisoxazole binds

90%.

The clinical importance of plasma proteins binding are as follows:

(a) The bound fraction of drugs is not available for action. This fraction is in equilibrium with free drug in plasma nad dissociates whin the concentration of

the latter is decreased due to elimination.

(b) If protein and drug binding is very high, then it makes the drug long acting, (c) One drug can bind to many sites of the protein albumin Opposite to it more than

one drug can bind to the same site.

Tissue storage : Drugs may also accumulate in specific organs or get bound to specific

tissue constituents. Some drugs may also bind to specific intracellular organelle, e.g., tetracycline

to mitochondria and chloroquine to nucleus. Certain drugs possess high toxicity because of

chloroquine on retina, emetine on heart and skeletal muscle, and tetracycline on bone and teeth.

2.2.1 Excretion and elimination

After absorption of drugs, they undergo the process of biotransformation, i.e., altered

chemically in the body, and thus form their metabolites which are excreted in the following way:

(a) Urine : (b) Faeces : (c) Exhaled air : Gases and volatile liquids such as alcohol, general

anaesthetics and paraldehyde etc. are eliminated by lungs.

23

(d) Saliva and sweat : In the excretion of drugs, the importance of sweat and saliva is negligible. However, potassium iodide, lithium, rifampin, heavy

metals and thiocyanates are excreted through this way.

(e) Milk: Most of the drugs enter in breast milk by passive diffusion, such as more lipid soluble and less protein bound the drugs.

Renal excretion : All water soluble drugs are excreted by kidney.

Glomerular filtration : In the capillaries of glomerular, larger pores are found which are

able to filter all non protein bound drugs. In renal failure of after the age of 50 glomerular

filtration rate decreases progressively.

Tubular reabsorption : Lipid soluble drugs filtered at the glomerulus diffuse back in the

tubules, because 99% of glomerular filtrate is reabsorbed, but non lipid soluble and highly

ionezed drugs are not able to do so. This occur by following way.

(a) Weak acids ionize more and are less reabsorbed in alkaline urine. (b) Weak bases ionize more and are less reabsorbed in acidic urine.

This principle is important to utilize for elimination of poisonous drugs.

Tubular secretion : This is the active transfer of organic acids and bases. Tubular

transport mechanisms are not well developed at birth. Duration of action in many drugs, e.g.

penicillin, aspirin cephalosporins etc. is longer in neonates. These systems mature during infancy.

2.2.2 Pharmacokinetics Of Elimination

Drug elimination is sum total of metabolic inactivation and excretion. The

pharmacokinetics of elimination of drug gives an idea to devise rational dosage regimens and to

modify them according to individual needs. There are three pharmacokinetic parameters, such as:

(a) Clearance (b) Bio availability (c) Volume of distribution

(a) Clearance : The clearance of a drug is the theoretical volume of plasma from which the

drug removed completely in unit time. Clearance (CL) can be expressed as

Rate of elimination of drug

CL = –––––––––––––––––––––

Plasma concentration (c)

(i) First order Kinetics : The rate of elimination of drug is directly proportional to

drug concentration while clearance remains constant, or a constant fraction of the

drug present in the body is eliminated in unit time.

(ii) Kinetics of drugs alter from first order to zero order at higher doses.

Plasma half-life : The plasma half-life of a drug is the time taken for its plasma

concentration to be reduced to half of its original value.

A drug which has one compartment distribution and first order of elimination, is plot is

drawn between plasma concentration and time, which shows two slopes:

(a) Due to distribution, initially declining an a-phase. Half-life, of the drugs. Plasma concentration : Time plot of a drug eliminated by first order

kinetics after intravenous injection elimination t½ is :

where in2=natural logarithm of 2 or 0.693

k= elimination rate constant of the drug i.e. the fractions of total amount of drug in the body

which are removed in per unit of time.

Clearance (CL)

K= ––––––––––––––––––––––––

Volume of distribution (V)

V

24

t½ = 0.693 x –––––

CL

Hence

For example, if 2g of a drug is present in the body and 0.1 g of it is eliminated every hour

then,

0.1 = 0.05

K= –––––––––––

2

The drugs can be eliminated from the body as :

1 -50% drug is eliminated from body

2 -75% (50+25) drug is eliminated

3 -87% (50+25+12.5) drug is eliminated

4 -93.75% (50+25+12.5+6.25) drug is eliminated

Repeated drug administration :

If thetherapeutic plasma concentration of the drug has been worked out and its clearance

is known the dose rate can be obtained as :

Dose rate = target Cpss x Clearance

target Cpss x clearance

Dose rate = ––––––––––––––––––––

Fraction

When drugs eliminated by first order kinetics, the dose rate Cpss relatonship is linear,

Some drugs follow Michaelis Menten kinetics, the elimination occurs by zero order kinetics over

the therapeutic range.

In this case, the rate of drug elimination is expressed as:

(V max)(C)

Rate of drug elimination = –––––––––––

K m + C

Target level strategy:

Loading dose : Loading dose is a single or few rapidly repeated doses which are given in

the beginning to attain target concentration, it may be expressed as:

Target plasma conecentration x Volume

Loading dose = –––––––––––––––––––––––––––––––

Fraction of drug (F)

Target plasmaconcentration x clearance (CL)

Maintenance dose rate = ––––––––––––––––––––––––––––––––––––

Fraction of dose (F)

Bioavailability of drug is 100% which is administered intravenously, but is low when

taken orally because:

(i) The drug may be absorbed partially. (ii) The absorbed drug may undergo first pass metabolism in intestinal wall, liver of

to be excreted in bile.

Oral formulation of a drug from different manufacturers of different batches from the

same manufacturer may have the same quantity of drug (chemically equivalent) but may not yield

same blood level, i.e. biologically inequivalent.

When a drug is taken in solid form, it must break into particles of the active drug, before

its absorption.

Tablets and capsules consist of a number of different materials such as binders,

lubricants, diluents, stabilizing agents etc. The nature of these and details of the manufacture

25

process, The rate of dissolution is governed by the particle size, solubility, crystal form etc. of the

physical properties of drug. Difference in bioavailability may be due to the change in dissolution

and disintegration rate.

2.4 Pharmacokinetics In Drug Development Process

To use a drug for longer time, it is generally advantageous to modify a drug by

following manner:

(i) By prolonging absorption from site of administration: (a) Oral : Drug particles are coated with resins, plastic materials etc. which disperse

release of the active ingredients in gastrointestinal tract.A semipermeable

membrane is used to control the release of drug from the bable of capsule.

(b) Parenteral : The subcutaneous and intramuscular injection of drug in insoluble form or as oily solution pallet implantation and biodegradable implants may

develop a drug action.

(c) Transdermal drug delivery : The drug which is used as ointment, in adhesive patches, or strips applied on skin is becoming popular.

(ii) By increasing plasma protein binding : Development of drugs have been made by increasing plasma proteins binding which may be slowly released in the free active form

e.g. sulphadoxine.

(iii) By retarding renal excretion: The tubular secretion of drug being an active process which can be reduced by a competing substance, for instance, probenecid prolongs time

of action of penicillin and ampicillin.

2.5 Introduction

Pharmacodynamics is the study of drug effects, attempts to elucidate the complete effect

of action, sequence and the dose effect relationship.

Modification of the effects of one drug by another drug and by other factors is also a part

of pharmacodynamics.

Drug Action:

(a) Stimulatio: (b) Depression: (c) Irritation: (d) Replacement: (e) Cytotoxic Action:

Mechanism of Drug Action : Drug Action mechanism is classified into four parts:

(a) Physical Action: (b) Chemical Action: (c) Through Enzymes:

2.5.1 enzyme stimulation

Drugs are truly foreing substances. Stimulation of enzymes by drugs is unusual. The

endogenous mediators and modulators stimulate the enzymes, e.g., pyridoxine acts as a cofactor

and increase decarboxylase activity similarly aderenaline stimulates adenylyl cyclase. Stimulation

of an enzyme enhances its affinity for the substrate, thus rate constant (Km) of the enzyme

reaction decreases.

Many drugs induce the microsomal enzymes e.g., enzyme penicillinase is obtained from

a mould, and is induced by methicillin.

Many insecticides, carcinogens, and drugs interact with DNA and increase the synthesis

of microsomal enzyme protein, particularly glucuronyl transferase and cytochrome P-450.

26

2.5.2 Enzyme Inhibition

Enzymes are inhibited generally by drugs. Inhibition is of two types:

(a) Non specific inhibition: Strong acids, heavy metal salts, phenols, alcohol, formaldehyde and alkalies inhibit enzymes non specifically. The chemicals and

drugs change the tertiary structure of enzymes and denature their protein portion

and thus inhibit them.

(b) Specific inhibition: Many drugs inhibit a specific enzyme without affecting others. This type of inhibition is categorised in two parts:

(i) Competitive Inhibition (Equilibrium Type) (ii) Non competitive Inhibition

In this type, the drug competes with the normal substrate or coenzyme to get a new

equilibrium. substrate concentration is increased sufficiently, it can displace the drug and the

same maximal reaction velocity can be obtained.

(A) Sulfonamides compete with para aminobenzoic acid (PABA) for bacterial folate synthetase.

(B) Carbidopa and methyldopa compete with levodopa for dopa decarboxylase. (C) Neostigmine and physostigmine compete with acetylcholine for cholinesterase. (D) A drug may also compete with coenzyme e.g., Warfarin competes with vitamin k

which acts as a coenzyme for enzyme which synthesize clotting factors in the

liver.

(iii) Noncompetitive inhibition: In noncompetitive inhibition, the inhibiters react with an adjacent site but not with the catalytic site of enzymes.

Inhibitor also changes the enzyme in such a way that it loses its catalytic

property. In such type of inhibition Km remains unchanged,

2.5.3 Sulphonamides :

In 1935, the daughter of Gerhand Domagk, a doctor working in a German dye factory,

suffered from severe streptococcal infection contracted from a pin prick. Domagk gave her an

oral dose of a dye called prontosil which had shown to inhibit the growth of streptocacci in mice.

Ernest fourneau in 1936 demonstrated that prontosil breaks down to produce sulphanilamide in

human body which is the actual active agent specifically lethal to streptococci.

2.5.4 Membrane Active Drugs

Membrane active drugs are volatile anesthetics and also known as general anesthetics.

General anesthetics are depressant drugs which produce partial or total loss of sense of pain, and

may be accompanied by loss of consciousness. This state of insensibility is known as anesthesia.

Membrane active drugs or general anesthetics act by depressing nervous function.

To administer gas or volatile liquid anesthetics, various equipment and techniques have

been used such as open drop method in which liquid anesthetic is dropped on a gauze of other

absorbent material supported on the patient's nose and mouth by a wire frame During the intake

of an anesthetic, its concentration in the blood quickly increases when the anesthetic moves

towards tissues, this concentration reaches in the arterial blood supply and thus the brain rapidly

acquires high concentration of anesthetic.

27

Types of membrane active drugs: A number of membrane active drugs are described

below.

Cyclopropane is also a currently used membrane active drug, but due to explosive nature

its use has declined now a days.

Ethers: Alkane, alkene, alkyne and alicylic ethers are potent membrane active drugs, but

only vinyl-substituted and ethyl-substituted ethers have been investigated as anesthetic durgs.

Chain length increases, the anesthetic activity of low-molecular-weight hydrocarbon ethers

increase. Divinyl ether and its analogs are not much important as anesthetic.

(2) Halogenated anesthetic agents: Introduction of halogen atoms (CI,Br,F) in membrane

active ethers increases anesthetic potency and decreases flammability. These drugs ars as

follows:

Fluorinated hydrocarbons: Fluorinated hydrocarbons such as flurorene,

methoxyflurane, isoflurane, and sevoflurane are developed as perfect anesthetic.

(3) Nitrous oxide: Nitrous oxide is a least potent and least toxic membrone active drug.

(4) Ketamine hydrochloride : Ketamine hydrochloride e.g., 2-(o-chlorophenyl)-2-methyl-

aminocyclo hexanone hydrochloride is a rapid-acting, potent, and a short duration

membrane active drug. accidental inhalation of trichloroethylene and 1, 1,1-

trichloroethane has been associated with brain damage,

2.5.5 Drug Metabolism

After the pharmacological response, the drugs are required then excreted from the body.

By enzymes of liver and various other tissue, the drugs may undergo a variety of chemical

changes.

2.5.6 The study of drug-metabolism and other xenobiotics,

Drug metabolism usually leads to detoxication, oxidation, reduction and other enzyme

catalyzed reactions, therefore, may form a metabolite having toxic or therapeutic effects. Thus

drugs and other chemicals such as some natural products, food additives, insecticides,

preservatives, environmental and agrochemicals etc, undergo enzymic transformation in the body,

which generally cause the loss of pharmacological activity.

Although liver is the major site of drug metabolism, however, some drug metabolizing

enzymes are also found in kidney, lung, plasma, nervous tissue and the gastrointerstnal tract.

Liver disease should have an important effect on the metabolism of drugs. The capacity

of drug metabolism is greatly affected in damaged or chronic diseased liver.

The ability of the liver to metabolize a substance in one pass is called 'first-pass effect or

presystemic hepatic elimination.

The liver can remove chemicals from the blood after their absorption from the

gastrointestinal tract.

The principal route of drugs and their metabolites excretion occurs in the urine. If drugs

and other compounds are not metabolized.

Urine is not the only route for excreting drugs and their metabolites from the animal

body. The other routes for excretion are:

(a) Bile (b) Saliva (c) Lungs (d) Sweat and (e) Milk The bile has been recognized as a major route of excretion for various exogenous and

endogenous substances.

28

Pathways Of Drug Metabolism

(1) Phase 1 Reaction : This is a biotransformation process and consists of oxidation, hydroxylation, reduction, and hydrolysis-enzymatic reactions. In phase 1

reactions, either a new functional group is introduced into the drug molecule or

an preexisting functional group undergoes modification. Hence, drug becomes

more polar and therefore it can be excreted more readily.

(2) Phase 2 Reaction: The phase 2 reactions are conjugation reactions. These are enzymatic synthesis in which a functional group is masked by the addition of

a new group. Such groups are glucuronic acid, certain amino acids, acetyl of

sulfate groups. These groups increase the polarity of the drug and caused rapid

excretion.

(3) Dealkylation of Ether and Thioether : By a hydroxylation of the sulphur and oxygen alkyl groups, an acetal or thio-acetal are formed.

Microsomal Reductions : For the metabolism of drugs, some enzymes are capable of

reducing azo and nitro groups of the drugs. These enzymes are found in microsomal systems. For

example, the nitro group of hypnotic benzodiazepine, nitrazepam, gets reduced into the 7-amino

derivative.

The most important of these has been alcohol dehydrogenase which catalyses the

oxidation of ethanol to acetaldehyde.

Hydrolysis : In the brain, Kidney, blood liver microsomes and many other tissues esters

and amides get hydrolysed by enzymes. The bulky esters get slowly hydrolysed and may often

get excreted unchanged or unhydrolysed.

Phase 2 Reactions-Conjugation

The added group helps in blocking the functional group as well as

decreasing the lipophility of the molecule, hence facilitating its excretion. Formation of

glucuronide is a most common encountered conjugation reaction.

Alcohols and phenols form ether type glucuronides, acids form acid-type glucuronide,

amines form N-glucuronides, while thiols give S-glucuronides. These glucuronides are more

soluble in water and are more acidic than the starting drug, hence at normal pH they are more

likely to be ionised and consequently even less lipophilic. Most of the elimination into the urine

occurs via kidney.

Glycine conjugation, acetylation and mercapturic acid formation are other types of

conjugations of lesser importance.

2.5.7 Biotransformation

Chemical changes of the drug in the body is called biotransformation. It is important to

convert non-polar, i.e., lipid soluble compounds into polar, i.e., lipid insoluble, so that they may

not be reabsorbed in the renal tubules and are excreted from the body. Many of the hydrophilec

drugs, for example, neostigmine, decamethonium and streptomyci etc. can not be biotransformed

and are excreted unchanged.

Types Of Biotransformation Reactions

(i) Non synthetic reactions (ii) Synthetic reactions

(i) Non Synthetic Reactions

Non synthetic reactions form metabolite which may be either active of inactive. These are

phase 1 reactions and can be classified as:

29

(A) Oxidation : Oxidation is the most important drug metabolizing reaction. This reaction involves addition of oxygen of removal of hydrogen. The examples are oxygenation at C,

N or S atoms, hydroxylation, N or O-dealkylation, oxidative deamination etc.

Generally oxidative reactions are occurred by a group of monooxygenases in the liver.

Phenothiazines, barbiturates, steroids, paracetamol; benzodiazepines, phenytoin,

theophylline and many other drugs are oxidized in this way. Rate of metabolism of drugs.

(1) CyP 3A 4/5 : About 50% drugs are blotransformed by this isoenzyme. It is available in liver, kidney and intestine.

Inhibitors : This is inhibited by many compounds such as clarithromycin,

erythromycin, itraconazole, verapamil etc.

(2) CYP 2 D6 : About 20% drugs get transformed by this isoenzyme. (3) CYP 2C 8/9 : Nearly > 15 commonly used drugs including narrow safety margin

drugs such as warfarin and phenytoin etc. are metabolixed by this enzyme.

(4) CYP 2 C 19 : This enzyme metabolizes about >12 frequently used drugs such as lansoprazole and omeprazole etc.

(ii) SYNTHETIC REACTIONS

These are phase 2 conjugation reactions. The metabolites formed by this

biotransformations are mostly inactive. Synthetic reactions have high energy requirement. These

involve conjugation of drug or its phase 1, metabolite with an endogenous substrate such as

amino acids, of carbohydrate.

(a) Glucuronide Conjugation : This is a most important synthetic reaction where a compound which contains hydroxyl or carboxylic acid group can be easily

conjugated with glucuronic acid.

(b) Acetylation : Compounds containing amino or hydrazine residues are conjugated with the help of acetyl coenzyme-A and show acetylation reaction e.g.

sulfonamides, hydralazine, p-amino sulfanilamide, isoniazid etc.

2.5.8 Significance Of Drug-Metabolism In Medicinal Chemistry

The metabolic changed drugs have been of considerable interest and of great practical

value in the search for new and improved medicines.

The azodye, prontosil, which is inactive in vitro, is converted in the body to the active

sulphanilamide by metabolic reduction process.

The metabolic acetylation of the sulphonamides served in the development of compounds

which are acetylated to a lesser extent and whose acetylated derivatives are more soluble,

hencereduce kidney damage to crystallization in the renal tubules.

Other significance of drug development related to metabolism is analgesic properties of

phenacetin, i.e., p-ethoxy acetanilide which depends on its coneversion by O-dealkylation to

produce an active metabolite, acetaminophen i.e., p-hydroxyacetanilide.

The antidepressant properties of imipramine and amitriptyline, both tertiary amines are

to be mediated by their secondary amine metabolites, called desipramine and notriptyline.

Cholroguanide (paludrine), 1 (p-chlorophenyl)-5-isopropylbiguanide, shows its

antimalarial activity only when it is converted into 1-(p-chlorophenyl)-2, 4-diamino-6-dimethyl-

dihydro-1, 3, 5-triazine